Frictional losses in the piston skirt to cylinder liner conjunction account for approximately 2.5% of
the energy supplied to the modern car [1]. These losses are contributed by viscous shear of the
lubricant and asperity interactions on the contiguous surfaces. However, for most of the piston cycle
the regime of lubrication is dominated by elastohydrodynamics or hydrodynamics. Hence, friction
due to viscous shear is dominant.
Most idealistic analyses employ a “cold” piston skirt shape and use either a measured profile or by
approximated polynomials as the input shape [2-4]. In reality, however, pistons are subject, not only
to contact forces, but also thermo-mechanical distortion. These are as the result of thermal
expansion of the piston as well as its global mechanical deformation in situ. They alter the pistonliner
conjunctional gap. The piston structure is designed in such a way as to prevent gross localised
wear in service by means of skirt profile and structural stiffness modification [5]. Considering the
combined effect of global as well as local deformation of the skirt under the influence of contact
force, it is vital to take into account the effect of shape and rigidity of both the piston and liner
structures in an integrated thermoelastic and elastohydrodynamic analysis. This approach is more
representative of the in situ “hot” skirt condition as noted by McClure [6].
This paper shows the significant differences observed in the generated pressures, film thickness and
friction by comparing “cold” piston profiles; disregarding large scale global deformation and “hot”
thermo-elastically deformed skirt conjunctions with representative skirt stiffness.